Exhaust systems in modern internal combustion engines may be provided with catalysts for catalytic decomposition of noxious exhaust gases, for example, those occurring in motor vehicles. For good functioning of the catalysts, air and fuel should be supplied to the engine in a predetermined ratio. For this purpose, engine controls are provided, which may be connected to a lambda probe. The signals of the lambda probe indicate the composition of the exhaust gases and thus permit the engine control to optimally regulate the ratio of fuel and combustion air for the catalyst.
In this regard, two concepts are described below.
The first concept attempts to achieve a stoichiometric combustion, i.e., when the quantity of oxygen in the combustion air corresponds exactly to the oxygen demand for complete combustion of the fuel supplied. In this case, the engine is not operated with an oxygen excess (λ>1) or an oxygen deficiency (λ<1). This type of operation may be characterized by λ=1.
For stoichiometric combustion, narrowband lambda probes may be sufficient for exhaust gas sensing, the Nernst electrode being acted upon essentially directly by the exhaust gas.
This engine control considers the effect that an electric voltage, which is generated by diffusion of oxygen ions and is detectable between the reference electrode and the Nernst electrode, changes in value in the range of λ=1. Thus, a signal that indicates a deviation from the desired operation of stoichiometric combustion is available. This signal may indicate both a deficiency of oxygen as well as an excess of oxygen, with respect to stoichiometric combustion.
Such sensors are referred to in German Published Patent Application No. 44 01 749, for example.
The second concept attempts to achieve operation of the combustion engine predominantly with an excess of oxygen in combustion, since this may allow a significant reduction in fuel consumption. However, harmful nitrogen oxides, which may form in combustion with excess oxygen, may be absorbed by storage catalysts in the exhaust line of an automotive engine only for a limited period of time. Before the storage capacity of these storage catalysts is depleted, operation of the engine must be switched briefly to combustion with an oxygen deficiency, to permit reduction of the nitrogen oxides previously stored in the catalyst. These nitrogen oxides may accumulate, for example, due to incompletely burned fuel constituents entering the exhaust gas line. The engine control should therefore be switched repeatedly and intermittently between a mode of operation that is predominant over a period of time in which the values of λ are above 1 and a relatively short-term mode of operation, in which the values of λ are less than 1.
Broadband lambda probes may be necessary for such intermittent operation with greatly varying values of λ.
With such lambda probes, the Nernst electrode is arranged in a separate chamber, which communicates with the exhaust gas stream via a diffusion zone in the body of the probe. In addition, an internal pump electrode, which is situated inside this chamber, which may be connected electrically to the Nernst electrode, and which cooperates with an external pump electrode through a solid electrolyte layer, is essentially directly exposed to the exhaust gas stream. If an external electric voltage is applied between the two pump electrodes, both of which may be gas-permeable in at least some areas, an oxygen ionic current is generated between the pump electrodes in a direction depending on the polarity of the applied voltage and with an amperage depending on the electric potential difference. This external voltage permits the control of the diffusion stream of the exhaust gases into the diffusion chamber, for example, by a regulator which adjusts the external electric voltage between the pump electrodes and the electric current occurring between the pump electrodes because of the oxygen ionic current, so that an electric voltage having a predetermined setpoint is maintained between the reference electrode and the Nernst electrode. Thus, the polarity and amperage of the electric current between the pump electrodes may produce a signal in correlation with the composition of the exhaust gases and thus with the λ values.
Such probes are referred to in German Published Patent Application No. 37 44 206, for example.
The probes described above should be heated during operation to generate a signal that may be analyzed. Therefore, lambda probes and other gas sensors may have an electric resistance heater, which in the case of a probe body formed by a laminate, may be situated on or between layers of the laminate.